1. Field of the Invention
The present invention relates to linear beam electron devices, and more particularly, to an electron gun which provides multiple convergent electron beamlets suitable for use in a multiple beam klystron.
2. Description of Related Art
Linear beam electron devices are used in sophisticated communication and radar systems which require amplification of a radio frequency (RF) or microwave electromagnetic signal. A conventional klystron is an example of a linear beam electron device used as a microwave amplifier. In a klystron, an electron beam originating from an electron gun is caused to propagate through a drift tube that passes across a number of gaps which define resonant cavities of the klystron. The electron beam is velocity modulated by an RF input signal introduced into one of the resonant cavities. The velocity modulation of the electron beam results in electron bunching due to electrons that have had their velocity increased gradually overtaking those that have been slowed. The traveling electron bunches represent an RF current in the electron beam, which induces electromagnetic energy into a subsequent one of the resonant cavities. The electromagnetic energy may then be extracted from the subsequent resonant cavity as an amplified RF output signal.
Ever since the invention of the klystron, it has been recognized that a klystron having multiple beams in a bundle of separate drift tubes would have certain advantages over a klystron having a single electron beam in a single drift tube. If the gaps of the klystron are formed by the ends of the multiple drift tube bundles facing each other in an aligned fashion, the electric fields across each gap would be stronger and more uniform than would be the fields across a gap of a single drift tube. In addition, electrons in one of the drift tubes would be isolated from electrons in other ones of the drift tubes, so the electron repulsive forces, referred to as debunching forces, would be less. In theory, a high current, low voltage, multiple beam klystron would yield the same efficiency and power as a conventional klystron having a single low current electron beam operating at a much higher voltage. Also, a multiple beam klystron could achieve much more bandwidth than a conventional klystron because the fringing capacitance and electric field around the bundle of drift tubes at each gap would be a smaller fraction of the useful electric field in the gap which interacts with the electrons.
Despite the potential advantages of multiple beam klystrons, such devices have only been adapted for certain low power or low frequency applications in which a convergent electron beam is not necessary. In these non-convergent devices, electron beam focusing is provided by immersing the electron gun and drift tubes in a strong magnetic field which guides the electrons along the magnetic flux lines to the drift tubes. In one such approach, an electron gun was provided with a plurality of individual cathodes placed side by side, though this electron gun proved to be impractical since the individual cathodes could not be made to operate simultaneously. In an alternative approach, an electron gun was provided with a plurality of electron emitting spots driven by a common heater. Multiple beam klystrons incorporating such an electron gun have demonstrated lower operating voltage for the same bandwidth and power as a conventional helix traveling wave tube amplifier, as well as higher efficiency without using a multi-staged depressed collector. Nevertheless, the non-convergent electron beams of the prior art devices have limited current density which prevent them from developing more power at higher frequencies. In view of the difficulty in forming a converging group of electron beams suitable for use in the bundle of drift tubes, a multiple beam klystron has not been adapted for high power operation.
Accordingly, it would be desirable to provide a convergent electron gun having a plurality of high current beamlets that could be focused into multiple drift tubes with reasonable current density at the cathode of the electron gun. Such an electron gun would permit construction of a multiple beam klystron that would provide high operating power at high frequencies.